I disagree, I always figured he saw the bogart in its natural shape - of course, it would have made sense for him to describe it for the text book writers n stuff, but this Moody after all and besides JKR isn't always perfect. Thing is, it's when the bogart
faces a new person (i.e. the nearest person) that it changes shape - it's not instantaneous and magical, it's the bogart itself that decides to transform.
Personally I imagine the bogart in its natural shape to be some slimy, fat green thing that's not very big
-TheQw